Introduction
Soybean is an economically important legume-cum-oilseed crop being rich
source of protein (40%) and edible oil (20%) (Chaudhary et al. ,
2015). However, soybean production is affected by a range of
environmental stresses (Sharmin et al. , 2020). In recent years,
increased occurrence of drought events was reported due to global
climate change (Bailey-Serres et al., 2012). Drought stress at
germination stage is one of the major constraints that badly affects
seed germination and seedling establishment. Accumulated evidence
indicates that yield is affected by drought stress at early (germination
and vegetative) and late (reproductive) growth stages (Zhao et
al ., 2017; Wijewardana et al. , 2019). However, a little progress
has been made to elucidate the genetic mechanisms of drought tolerance
during germination stage in soybean. Hence, concerted efforts are needed
to understand the genetic basis and genes associated with drought
tolerance during germination stage.
During germination stage many essential physiological and biochemical
processes are induced to initiate germination such as hydrolysis,
subcellular structures, cell elongation biosynthesis of macromolecules
and respiration (Bewley et al ., 2013). Many phytohormones i.e.,
ethylene (ETH), auxin (AUX), abscisic acid (ABA), gibberellic acid (GA),
salicylic acid (SA) and jasmonic acid (JA) as well as some environmental
conditions, including light, oxygen and temperature regulates seed
germination (Miransari and Smith, 2014). Drought stress is an important
environmental factor hampering seed germination and seedling vigor
(Delachiave et al. , 2003; Ahmad et al. , 2009). It not only
hinder the seed germination but also increases mean germination time
(Valliyodan and Nguyen, 2006). At germination, soybean seed must absorb
water (50% of its weight) to begin normal germination. Insufficient
water availability especially during seed emergence stage lower the
soybean yield (Wijewardana et al. , 2019; Kobraee et al. ,
2011). Several researchers have reported the negative impact of drought
on seed germination and seedling vigor in different crops such as mung
bean (Rani et al. , 2018), barnyard grass (Wu et al. ,
2019), sesame (Boureima et al . 2011), maize ( Ahmad et
al. , 2018) and soybean (Vieira et al . 1991).
Response of crop plants to drought stress at molecular level involve
perception, signal transduction, gene expression and ultimately
metabolic changes resulting in stress tolerance (Huang et al. ,
2012). Drought tolerance being a complex quantitative trait involves
numerous regulatory and functional genes (Jiménez et al. , 2013).
Hence, numerous genes are involved in stomatal movement, osmolyte
metabolism, antioxidant activity and phytohormones signalling, under
drought stress and re-watering (Shinozaki and Yamaguchi-Shinozaki, 2007;
Laxa et al. , 2019). Besides, drought stress stimulates many
adaptive signaling pathways to cope with adverse effects of stress, such
as MAPK cascade (Sinha et al. , 2011) and Ca2+signaling (Wilkins et al. , 2016) and signaling of phytohormones
(Tiwari et al. , 2017). These stress signaling pathways modify the
expression of drought responsive genes and provide defensive mechanisms
through activating downstream TFs, (Lan et al. , 2017) However,
most of these gene function as well as regulatory networks involved in
drought stress at germination stage have remained elusive. Hence, it is
prerequisite to identify the genetic mechanism and networks involved in
modulation of drought tolerance.
Advances in next-generation sequencing (NGS) platforms i.e.,
Illumina/Solexa has made it possible to understand the complexity and
regulation of gene expression networks in different crop species under
dehydration stress (Chen et al. , 2016). The NGS platforms
provides wider and more robust transcriptome analyses (Severin et
al. , 2010). Transcriptome studies (with Affymetrix/RNA sequencing
technology) utilizing contrasting lines to understand drought stress
response has been carried in different crops such as rice (Degenkolbeet al . 2009; Lenka et al. , 2011), but in case of soybean
mostly single genotype was used (Le et al. 2012; Song et al. ,
2016). Use of contrasting genotypes in transcriptome studies will
provide comparative and detailed information on DEGs, that could
possibly assist in identifying important genes regulating drought
tolerance. Comparative transcriptomic analysis of various genotypes will
be helpful to dissect biological pathways and mechanisms imparting
tolerance to environmental stresses, as well as to explore genes under
selection. Although, significant insights have been provided by
fundamental research into the physiological and molecular responses of
plants to drought stress, but the divergence in root transcriptome of
soybean drought responsive genotypes during germination remains largely
unexplored.
In the present study, the comparative transcriptomic analysis between
drought-tolerant (PI342618B/DTP & A214/DTL) and drought-sensitive
(NN86-4/DSP & A195/DSL) soybean genotypes were analyzed comprehensively
to gain insights into soybean defense response under drought stress at
germination stage. This study provided an in-depth information at the
global transcriptome level, and identified pathways and candidate genes
associated with drought tolerance at germination stage in soybean.